Weed control strategies

Crop weed control requires combination of strategies

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Farmers are always looking for ways to combat barley grass, and have upgraded their weed control strategies. Bob Freebairn outlines some of the ways.

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Barley grass September 2019 in a grazed-out oat crop. Becoming a troublesome weed in dual purpose oats.

Barley grass September 2019 in a grazed-out oat crop. Becoming a troublesome weed in dual purpose oats.

Barley grass can become an issue in many winter crops, especially early sown dual purpose cereals.

Our property is no exception and we have had to upgrade weed control strategy to address a potentially major crop performance reduction issue. A multifaceted approach is generally best for any weed control strategy and that is what we are endeavouring to do.

Oats in the same paddock 2020 with excellent control of barley grass. Keys were earlier sowing, weed suppression herbicide, and competitive crop with higher nitrogen.

Oats in the same paddock 2020 with excellent control of barley grass. Keys were earlier sowing, weed suppression herbicide, and competitive crop with higher nitrogen.

Quite a bit of GRDC funded research suggests sowing as early as possible within a crops sowing window, provided varieties are suited to that window, is a good starting point for best weed control.

In the case of dual purpose oats we have endeavoured to sow earlier than is generally accepted as desirable, early March for our district (typical central west environment). This year we sowed on February 25.

Early sowing, provided soil moisture is good, with a winter habit variety, which means the crop won't prematurely go to head, allows for rapid crop germination and quick establishment. This commonly allows the crop to establish ahead of weeds like barley grass and get a good competitive advantage over them.

Dual purpose brassica crops add extra control options for grass weeds like barley grass. Cattle health issues can be an extra problem with these crops however.

Dual purpose brassica crops add extra control options for grass weeds like barley grass. Cattle health issues can be an extra problem with these crops however.

An old and cheap herbicide chlorsulfuron was applied to the crop at the two leaf stage. While chlorsulfuron is not registered for barley grass control (but is for many other weeds) it commonly has a useful role in reducing impact of the weed, especially if a light to moderate rain event falls relatively soon after application.

A third important weed control aspect is a crop that is as vigorous as possible to maximise its competitiveness against weeds. High soil fertility is clearly important for maximum crop vigour. We have done nothing special in this aspect, sown with 75kg/ha DAP and top-dressed relatively early with 100kg/ha urea. Soil testing is a good guide for assessing most likely best economical fertiliser rates.

Some research also shows higher seed rates and closer row spacing helps crop competitiveness against weeds. Normal sowing rates for oats in slopes environments is around 60 kg/ha and 90 kg/ha for tableland areas. At least adhering to these helps and going a bit higher won't hurt. It's difficult for many to change row spacing but as close as possible to the old 17.5cm (seven inches) is desirable.

Research in crops like canola show hybrid seed, because of higher seedling vigour, is commonly far more competitive than open pollinated varieties. Also within a variety grading out larger seeds are more competitive than smaller seeds against weeds.

Coming out of a good spring, as it is likely to be for many districts this year, is commencing fallows early before likely problem weeds seed down. This especially applies when coming from a pasture phase to a cropping phase.

Prevention of weed seed set in the year prior to seed set is a good control starting point. For many areas this option has already passed for early flowering weeds like barley grass.

Further herbicide options for barley grass control is to replace oats with winter wheat or triticale for both pre-emergent or post-emergent herbicide applications. A number of herbicides can be used on these crops but are not registered in oats. Using brassica crops, such as canola, provides even more herbicide options, including even greater herbicide choice for Clearfield varieties. For long term cropping programs herbicide choice is increasingly important to avoid weeds developing resistance to them.

Back to our example barley grass looked like being a developing issue but integrated control has worked well this year. Being a cattle only livestock business we are a bit wary of using brassica in our system as a weed control option. In some years, like the present one, several cattle producers have suffered losses and other health issues.

Talking with livestock specialists and veterinarians it seems brassica health issues can be complex, especially with cattle, and include other than nitrate poisoning, pulpy kidney and bloat (all common with many crops) aspects. This is not my level of expertise and my suggestion is seek good advice before including brassica crops in dual purpose use with livestock, especially cattle.

Next week: Passing of pioneer serradella grower. Serradella was first discovered as a light acid soil pasture legume.

  • Bob Freebairn is an agricultural consultant based at Coonabarabran. Email robert.freebairn@bigpond.com or contact 0428 752 149.
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